Compositionally-graded band gap heterojunction solar cell

ABSTRACT

A photovoltaic device includes a composition modulated semiconductor structure including a p-doped first semiconductor material layer, a first intrinsic compositionally-graded semiconductor material layer, an intrinsic semiconductor material layer, a second intrinsic compositionally-graded semiconductor layer, and an n-doped first semiconductor material layer. The first and second intrinsic compositionally-graded semiconductor material layers include an alloy of a first semiconductor material having a greater band gap width and a second semiconductor material having a smaller band gap with, and the concentration of the second semiconductor material increases toward the intrinsic semiconductor material layer in the first and second compositionally-graded semiconductor material layers. The photovoltaic device provides an open circuit voltage comparable to that of the first semiconductor material, and a short circuit current comparable to that of the second semiconductor material, thereby increasing the efficiency of the photovoltaic device.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.12/849,966, filed Aug. 4, 2010 the entire content and disclosure ofwhich is incorporated herein by reference.

BACKGROUND

The present disclosure relates to photovoltaic devices, and moreparticularly to photovoltaic devices including a compositionally-gradedband gap heterojunction and methods of forming the same.

A photovoltaic device is a device that converts the energy of incidentphotons to electromotive force (e.m.f.). Typical photovoltaic devicesinclude solar cells, which are configured to convert the energy in theelectromagnetic radiation from the Sun to electric energy. Each photonhas an energy given by the formula E=hv, in which the energy E is equalto the product of the Plank constant h and the frequency v of theelectromagnetic radiation associated with the photon.

Referring to FIG. 1, the functionality of a photovoltaic device can beapproximated by an equivalent circuit that includes a current source, adiode, and two resistors. The circuit of FIG. 1 approximates a unit areaof a physical photovoltaic device, which provides electrical currentthat is proportional to the total irradiated area of the physicalphotovoltaic device. The photovoltaic current per unit area generated bythe physical photovoltaic device is referred to as a short-circuitcurrent density J_(sc), i.e., the current density generated by thephysical photovoltaic device if the positive node and the negative nodeof the physical photovoltaic device are electrically shorted. Thus, thecurrent source in FIG. 1 generates an electrical current with a currentdensity of the short-circuit current density J_(sc).

Power dissipation through internal leakage current is approximated by ashunt resistance R_(sh). A finite value for the shunt resistance R_(sh)triggers an internal leakage current through a physical photovoltaicdevice, and degrades the performance of the physical photovoltaicdevice. The lesser the shunt resistance R_(sh), the greater is theinternal power loss due to the internal leakage current.

Power dissipation through internal resistance of the physicalphotovoltaic device is approximated by a series resistance R_(s). Anon-zero value for the series resistance R_(s) triggers Joule losswithin the physical photovoltaic device. The greater the seriesresistance R_(s), the greater is the internal power loss due to theinternal resistance of the physical photovoltaic device.

The potential difference between the positive node and the negative nodeof a photovoltaic device generates an internal current that flows in theopposite direction to the photocurrent, i.e., the current represented bythe current source having the short-circuit current density J_(sc). Thedark current has the same functional dependence on the voltage acrossthe current source as a diode current. Thus, the dark current isapproximated by a diode that allows a reverse-direction current. Thedensity of the dark current, i.e., the dark current per unit area of thephysical photovoltaic device, is referred to as the dark current densityJ_(dark). An external load can be attached to an outer node of theseries resistor and one of the nodes of the current source. In FIG. 1,the value the impedance of the load is the value of the actual impedanceof a physical load is divided by the area of the physical photovoltaiccell because the equivalent circuit of FIG. 1 describes thefunctionality of a unit area of the physical photovoltaic cell.

Referring to FIG. 2, a schematic graph of an I-V curve of a physicalphotovoltaic device structure is shown. The bias voltage V is thevoltage across the load in the equivalent circuit of FIG. 1. The opencircuit voltage Voc corresponds to the voltage across the load as theresistance of the load diverges to infinity, i.e., the voltage acrossthe current source when the load is disconnected. The inverse of theabsolute value of the slope of the I-V curve at V=0 and J=J_(sc) isapproximately equal to the value of the shunt resistance R_(sh). Theinverse of the absolute value of the slope of the I-V curve at V=V_(oc)and J=0 is approximately equal to the value of the series resistanceR_(s). The effect of the dark current is shown as an exponentialdecrease in the current density J as a function of the bias voltage Varound a non-zero value of the bias voltage.

The operating range of a photovoltaic device is the portion of the I-Vcurve in the first quadrant, i.e., when both the bias voltage V and thecurrent density J are positive. The power density P, i.e., the densityof power generated from a unit area of the physical photovoltaic deviceof FIG. 1, is proportional to the product of the voltage V and thecurrent density J along the I-V curve. The power density P reaches amaximum at a maximum power point of the I-V curve, which has the biasvoltage of V_(m) and the current density of J_(m). The fill factor FF isdefined by the following formula:

$\begin{matrix}{{FF} = {\frac{J_{m} \times V_{m}}{J_{sc} \times V_{oc}}.}} & \left( {{Eq}.\mspace{14mu} 1} \right)\end{matrix}$

The fill factor FF defines the degree by which the I-V curve of FIG. 3approximates a rectangle. The fill factor FF is affected by the seriesresistance R_(s) and the shunt resistance R_(sh). The smaller the seriesresistance R_(s), the greater the fill factor FF. The greater the shuntresistance R_(sh), the greater the fill factor FF. The theoreticalmaximum for the fill factor is 1.0.

The efficiency η of a photovoltaic device is the ratio of the powerdensity at the maximum power point to the incident light power densityP_(s). In other words, the efficiency η is given by:

$\begin{matrix}{\eta = {\frac{J_{m} \times V_{m}}{P_{s}}.}} & \left( {{Eq}.\mspace{14mu} 2} \right)\end{matrix}$

Eq. 2 can be rewritten as:

$\begin{matrix}{\eta = {\frac{J_{sc} \times V_{oc} \times {FF}}{P_{s}}.}} & \left( {{Eq}.\mspace{14mu} 3} \right)\end{matrix}$

Thus, the efficiency h of a photovoltaic device is proportional to theshort circuit current density J_(sc), the open circuit voltage V_(oc),and the fill factor FF.

The efficiency η of a photovoltaic device depends on the spectralcomposition of the incident light. For solar cells, the efficiency iscalculated under a standard radiation condition defined as 1 sun, whichemploys the spectrum of the sunlight.

As Eq. 3 indicates, the efficiency η of a photovoltaic device isproportional to the product of the short circuit current density J_(sc)and the open circuit voltage V_(oc). In order to enhance the efficiencyη of a photovoltaic device, therefore, it is necessary to increase theproduct of the short circuit current density J_(sc) and the open circuitvoltage V_(oc).

BRIEF SUMMARY

A photovoltaic device includes a composition modulated semiconductorstructure, which includes a p-doped first semiconductor material layer,a first intrinsic compositionally-graded semiconductor material layer,an intrinsic semiconductor material layer, a second intrinsiccompositionally-graded semiconductor layer, and an n-doped firstsemiconductor material layer. Each of the first and second intrinsiccompositionally-graded semiconductor material layers includes an alloyof a first semiconductor material having a greater band gap width and asecond semiconductor material having a smaller band gap with, and theconcentration of the second semiconductor material increases towards theintrinsic semiconductor material layer in the first and second intrinsiccompositionally-graded semiconductor material layers. The photovoltaicdevice provides an open circuit voltage comparable to that of the firstsemiconductor material, and a short circuit current comparable to thatof the second semiconductor material, thereby increasing the efficiencyof the photovoltaic device.

In one aspect, a photovoltaic device is provided, which includes astack, from top to bottom, of a first intrinsic compositionally-gradedsemiconductor layer including a first compositionally-graded alloy of afirst semiconductor material and a second semiconductor material, anintrinsic semiconductor material layer including at least the secondsemiconductor material, and an second intrinsic compositionally-gradedsemiconductor layer including a second compositionally-graded alloy ofthe first semiconductor material and the second semiconductor material,wherein an atomic concentration of the second semiconductor materialmonotonically decreases with distance from an interface with theintrinsic semiconductor layer in the first compositionally-graded alloyand the second compositionally-graded alloy.

In another aspect, a method of forming a photovoltaic device isprovided. The method includes: forming a first intrinsiccompositionally-graded semiconductor layer on a substrate, the firstintrinsic compositionally-graded semiconductor layer including a firstcompositionally-graded alloy of a first semiconductor material and asecond semiconductor material; forming an intrinsic semiconductormaterial layer on the first intrinsic compositionally-gradedsemiconductor layer, the intrinsic semiconductor material layerincluding at least the second semiconductor material; and forming asecond intrinsic compositionally-graded semiconductor layer on theintrinsic semiconductor material layer, the second intrinsiccompositionally-graded semiconductor layer including a secondcompositionally-graded alloy of the first semiconductor material and thesecond semiconductor material. An atomic concentration of the secondsemiconductor material monotonically decreases, and may strictlydecrease, with distance from an interface with the intrinsicsemiconductor layer in the first compositionally-graded alloy and thesecond compositionally-graded alloy, and the second conductivity type isthe opposite of the first conductivity type.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is an equivalent circuit for a prior art photovoltaic devicestructure.

FIG. 2 is a schematic graph of an I-V curve of a prior art photovoltaicdevice structure.

FIG. 3 is a band diagram of a first hypothetical semiconductor structureincluding a p-n junction between an n-doped first semiconductor materialand a p-doped second semiconductor material in which the band gap of then-doped first semiconductor material is greater than the band gap of thep-doped second semiconductor material.

FIG. 4 is a band diagram of a second hypothetical semiconductorstructure including a p-n junction between a p-doped first semiconductormaterial and an n-doped second semiconductor material in which the bandgap of the p-doped first semiconductor material is greater than the bandgap of the n-doped second semiconductor material.

FIG. 5 is a band diagram of a third hypothetical semiconductor structureincluding a p-n junction within a compositionally-graded compositionsemiconductor layer located between an n-doped first semiconductormaterial layer and a p-doped second semiconductor material layer. Theconcentration of a first semiconductor material and a concentration of asecond semiconductor material are complimentarilycompositionally-graded. The band gap of the n-doped first semiconductormaterial is greater than the band gap of the p-doped secondsemiconductor material.

FIG. 6 is a band diagram of a fourth hypothetical semiconductorstructure including a p-n junction within a compositionally-gradedcomposition semiconductor layer located between a p-doped firstsemiconductor material layer and an n-doped second semiconductormaterial layer. The concentration of a first semiconductor material anda concentration of a second semiconductor material are complimentarilycompositionally-graded. The band gap of the p-doped first semiconductormaterial is greater than the band gap of the n-doped secondsemiconductor material.

FIG. 7 is a vertical cross-sectional view of a fifth hypotheticalsemiconductor structure including a p-i-n junction. The fifthhypothetical semiconductor structure includes a first material junctionbetween a p-doped first semiconductor material layer and an intrinsicsecond semiconductor material layer and a second material junctionbetween the intrinsic semiconductor material layer and an n-dopedsemiconductor material layer.

FIG. 8 is a band diagram of the fifth hypothetical semiconductorstructure. An intrinsic semiconductor layer having a homogeneouscomposition including intrinsic germanium forms an interface with ap-doped silicon region on one side and another interface with an n-dopedsilicon region on the opposite side.

FIG. 9 is a vertical cross-sectional view of an exemplary semiconductorstructure including a first intrinsic compositionally-gradedsemiconductor layer including a compositionally-graded composition of afirst semiconductor material and a second semiconductor material andhaving a doping of a first conductivity type, a second intrinsiccompositionally-graded semiconductor layer including acompositionally-graded composition of the first semiconductor materialand the second semiconductor material and having a doping of a secondconductivity type which is the opposite of the first conductivity type,and an intrinsic second semiconductor layer in between.

FIG. 10 is a band diagram for the exemplary semiconductor structure ofFIG. 9. All band gap grading is effected by a grading of the compositionof semiconductor materials, and is not affected by electrical dopants,i.e., p-type dopants or n-type dopants.

FIGS. 11A-11C are sequential vertical cross-sectional views of theexemplary semiconductor structure during various stages of amanufacturing process.

DETAILED DESCRIPTION

As stated above, the present disclosure relates to photovoltaic devicesincluding a compositionally-graded band gap heterojunction and methodsof forming the same, which are now described in detail with accompanyingfigures. Throughout the drawings, the same reference numerals or lettersare used to designate like or equivalent elements. The drawings are notnecessarily drawn to scale. It is noted that a stack, from top tobottom, of elements can be rotated to become a stack, from bottom totop, of the same elements or a lateral stack, from one side to anotherside, of the same elements.

As used herein, a crystal structure is “microcrystalline” if the averagegrain size of the material is from 1 nm to 10 microns.

As used herein, a “hydrogenated” semiconductor material is asemiconductor material including incorporated hydrogen therein, whichneutralizes dangling bonds in the semiconductor material and allowscharge carriers to flow more freely.

As used herein, a “semiconductor material” refers to a material having aresistivity from 10⁻³ Ohm-cm to 10⁸ Ohm-cm, and includes elemental andcompound semiconductor materials.

As used herein, an element is “optically transparent” if the element istransparent in the visible electromagnetic spectral range having awavelength from 400 nm to 800 nm.

As used herein, a quantity is “monotonically increasing” if saidquantity remains the same or increases with any positive change in anunderlying parameter, such as a distance from a particular point. Inthis case, said quantity is a monotonically increasing function of saidunderlying parameter in general mathematical terms.

As used herein, a quantity is “monotonically decreasing” if saidquantity remains the same or decreases with any positive change in anunderlying parameter, such as a distance from a particular point. Inthis case, said quantity is a monotonically decreasing function of saidunderlying parameter in general mathematical terms.

As used herein, a quantity is “strictly increasing” if said quantityincreases with any positive change in an underlying parameter, such as adistance from a particular point. In this case, said quantity is astrictly increasing function of said underlying parameter in generalmathematical terms.

As used herein, a quantity is “strictly decreasing” if said quantitydecreases with any positive change in an underlying parameter, such as adistance from a particular point. In this case, said quantity is astrictly decreasing function of said underlying parameter in generalmathematical terms.

As used herein, an element is “compositionally-graded” if a compositionof said element is monotonically increasing or monotonically decreasingthroughout said element and if said composition of said element isdifferent at one end compared with said composition of said element atanother end.

On one hand, the band gap of germanium is about one half of the band gapof silicon having a comparable phase. For example, crystalline germaniumhas a band gap of about 0.67 eV, and crystalline silicon has a band gapof about 1.10 eV. Hydrogenated amorphous germanium has a band gap fromabout 0.8 eV to about 0.9 eV, while hydrogenated amorphous silicon has aband gap from about 1.7 eV to about 1.8 eV. Because the open circuitvoltage of a photovoltaic device employing a homogeneous semiconductormaterial including a p-n junction or a p-i-n junction is approximatelyproportional to the band gap of the homogeneous semiconductor material,a photovoltaic device employing silicon provides a greater open circuitvoltage than a photovoltaic device employing germanium of the samecrystalline quality.

On the other hand, short circuit current of a photovoltaic device tendsto increase in a photovoltaic device employing a semiconductor materialhaving a smaller band gap. For example, the absorption coefficient ofgermanium is about two orders of magnitude greater than the absorptioncoefficient of silicon. Thus, a thinner germanium layer can absorb thesame energy from solar radiation as a significantly thicker siliconlayer. Further, germanium has higher collection efficiency than siliconbecause germanium is capable of absorbing a wider range of solarspectrum than silicon having the same crystalline quality. For example,single crystalline germanium having a band gap of 0.67 eV can absorbsolar spectrum with wavelength less than 1,852 nm, while singlecrystalline silicon having a band gap of 1.10 eV can absorb solarspectrum with wavelength less than 1,128 nm. Hydrogenated amorphousgermanium having a band gap of 0.85 eV can absorb solar spectrum withwavelength less than 1,460 nm, while hydrogenated amorphous siliconhaving a band gap of 1.75 eV can absorb solar spectrum with wavelengthless than 709 nm.

The higher open circuit voltage of silicon provides an advantage overthe lower open circuit voltage of germanium, while the higher shortcircuit current of germanium provides an advantage over the lower shortcircuit current of silicon. Combining the advantageous characteristicsof silicon and germanium, however, is not straightforward, and typicallyresult in decompositionally-graded performance.

Referring to FIG. 3, a band diagram is shown for a first hypotheticalsemiconductor structure, which includes a p-n junction between ann-doped first semiconductor material and a p-doped second semiconductormaterial. In the first hypothetical semiconductor structure, the firstsemiconductor material is single crystalline silicon and the secondsemiconductor material is single crystalline germanium. The verticalscale represents the energy level as measured from the vacuum level. Thefirst semiconductor material has a greater band gap than the secondsemiconductor material.

The portion of the band diagram with negative x coordinates, i.e., theportion to the left of the point marked with “0,” corresponds to ap-doped germanium region. The portion of the band diagram with positivex coordinates, i.e., the portion to the right of the point marked with“0,” corresponds to an n-doped silicon region. Although the p-typedopant concentration is substantially constant in the p-doped germaniumregion, the p-n junction causes the valence band and the conduction bandof the p-doped germanium region to bend downward near the p-n junction.The p-doped germanium region includes only germanium and electricaldopants, which are primarily p-type (except for insignificant quantityof n-type dopants near the p-n junction that are introduced into thep-doped germanium region by bulk diffusion). Although the n-type dopantconcentration is substantially constant in the n-doped silicon region,the p-n junction causes the valence band and the conduction band of then-doped silicon region to bend downward near the p-n junction. Then-doped silicon region includes only silicon and electrical dopants,which are primarily n-type (except for insignificant quantity of p-typedopants near the p-n junction that are introduced into the n-dopedsilicon region by bulk diffusion). The flat line around the energy levelof −4.3 eV corresponds to the Fermi energy level in the firsthypothetical semiconductor structure.

The open circuit voltage of the first hypothetical semiconductorstructure is determined by the external voltage to flatten either of thevalence band or the conduction band because flattening either bandinduces flow of electrical charges in the flattened band. Thus, the opencircuit voltage of the first hypothetical semiconductor structure isapproximately equal to the open circuit voltage of single crystallinegermanium, i.e., the second semiconductor material. Consequently, thefirst hypothetical semiconductor structure does not provide theadvantage of the open circuit voltage of the first semiconductormaterial, i.e., single crystalline silicon.

Referring to FIG. 4, a band diagram is shown for a second hypotheticalsemiconductor structure, which includes a p-n junction between a p-dopedfirst semiconductor material and an n-doped second semiconductormaterial. In the second hypothetical semiconductor structure, the firstsemiconductor material is single crystalline silicon and the secondsemiconductor material is single crystalline germanium. The verticalscale represents the energy level as measured from the vacuum level. Thefirst semiconductor material has a greater band gap than the secondsemiconductor material.

The portion of the band diagram with negative x coordinates, i.e., theportion to the left of the point marked with “0,” corresponds to ap-doped silicon region. The portion of the band diagram with positive xcoordinates, i.e., the portion to the right of the point marked with“0,” corresponds to an n-doped germanium region. Although the p-typedopant concentration is substantially constant in the p-doped siliconregion, the p-n junction causes the valence band and the conduction bandof the p-doped silicon region to bend downward near the p-n junction.The p-doped silicon region includes only silicon and electrical dopants,which are primarily p-type (except for insignificant quantity of n-typedopants near the p-n junction that are introduced into the p-dopedsilicon region by bulk diffusion). Although the n-type dopantconcentration is substantially constant in the n-doped germanium region,the p-n junction causes the valence band and the conduction band of then-doped germanium region to bend downward near the p-n junction. Then-doped germanium region includes only germanium and electrical dopants,which are primarily n-type (except for insignificant quantity of p-typedopants near the p-n junction that are introduced into the n-dopedgermanium region by bulk diffusion). The flat line around the energylevel of −4.55 eV corresponds to the Fermi energy level in the firsthypothetical semiconductor structure.

The open circuit voltage of the second hypothetical semiconductorstructure is determined by the external voltage to flatten either of thevalence band or the conduction band because flattening either bandinduces flow of electrical charges in the flattened band. The opencircuit voltage of the second hypothetical semiconductor structure isapproximately equal to the open circuit voltage of single crystallinegermanium, i.e., the second semiconductor material. Consequently, thesecond hypothetical semiconductor structure does not provide theadvantage of the open circuit voltage of the first semiconductormaterial, i.e., single crystalline silicon.

Referring to FIG. 5 a band diagram is shown for a third hypotheticalsemiconductor structure, which includes a p-n junction located in acompositionally-graded composition semiconductor layer. In the thirdhypothetical semiconductor structure, the first semiconductor materialis single crystalline silicon and the second semiconductor material issingle crystalline germanium. The vertical scale represents the energylevel as measured from the vacuum level. The first semiconductormaterial has a greater band gap than the second semiconductor material.

The right side of the band diagram including a flat valence band portionand a flat conduction band portion corresponds to an n-doped siliconregion. The left side of the band diagram including another flat valenceband portion and another flat conduction band portion corresponds to ap-doped germanium region. The sloped portion of the band diagramcorresponds to a compositionally-graded composition semiconductor layerincluding a p-n junction. In the portion of the compositionally-gradedcomposition semiconductor region to the right of the p-n junction, then-type dopant concentration is substantially constant, and can be thesame as the n-type dopant concentration in the n-doped silicon region.In the portion of the compositionally-graded composition semiconductorregion to the left of the p-n junction, the p-type dopant concentrationis substantially constant, and can be the same as the p-type dopantconcentration in the p-doped germanium region. Within thecompositionally-graded semiconductor region, the concentration ofgermanium increases strictly with distance from the interface betweenthe compositionally-graded semiconductor region and the n-doped siliconregion.

The open circuit voltage of the third hypothetical semiconductorstructure is determined by the external voltage to flatten either of thevalence band or the conduction band because flattening either bandinduces flow of electrical charges in the flattened band. Thus, the opencircuit voltage of the third hypothetical semiconductor structure isapproximately equal to the open circuit voltage of single crystallinegermanium, i.e., the second semiconductor material. Consequently, thethird hypothetical semiconductor structure does not provide theadvantage of the open circuit voltage of the first semiconductormaterial, i.e., single crystalline silicon.

Referring to FIG. 6, a band diagram is shown for a fourth hypotheticalsemiconductor structure, which includes a p-n junction located in acompositionally-graded composition semiconductor layer. In the fourthhypothetical semiconductor structure, the first semiconductor materialis single crystalline silicon and the second semiconductor material issingle crystalline germanium. The vertical scale represents the energylevel as measured from the vacuum level. The first semiconductormaterial has a greater band gap than the second semiconductor material.

The left side of the band diagram including a flat valence band portionand a flat conduction band portion corresponds to a p-doped siliconregion. The right side of the band diagram including another flatvalence band portion and another flat conduction band portioncorresponds to an n-doped germanium region. The sloped portion of theband diagram corresponds to a compositionally-graded compositionsemiconductor layer including a p-n junction. In the portion of thecompositionally-graded composition semiconductor region to the right ofthe p-n junction, the n-type dopant concentration is substantiallyconstant, and can be the same as the n-type dopant concentration in then-doped germanium region. In the portion of the compositionally-gradedcomposition semiconductor region to the left of the p-n junction, thep-type dopant concentration is substantially constant, and can be thesame as the p-type dopant concentration in the p-doped silicon region.Within the compositionally-graded semiconductor region, theconcentration of germanium increases strictly with distance from theinterface between the compositionally-graded semiconductor region andthe p-doped silicon region.

The open circuit voltage of the fourth hypothetical semiconductorstructure is determined by the external voltage to flatten either of thevalence band or the conduction band because flattening either bandinduces flow of electrical charges in the flattened band. Thus, the opencircuit voltage of the fourth hypothetical semiconductor structure isapproximately equal to the open circuit voltage of single crystallinegermanium, i.e., the second semiconductor material. Consequently, thefourth hypothetical semiconductor structure does not provide theadvantage of the open circuit voltage of the first semiconductormaterial, i.e., single crystalline silicon.

Referring to FIG. 7, a fifth hypothetical semiconductor structure is aphotovoltaic device structure, which incorporates some inventivefeatures of the present disclosure, i.e., the features ofcompositionally-graded-doping regions located near a p-i interface andan i-n interface. Specifically, the fifth hypothetical semiconductorstructure includes a stack, from top to bottom, of a substrate 10, acontact material layer 20, a p-doped first semiconductor material layer30, an intrinsic second semiconductor material layer 40′, an n-dopedfirst semiconductor layer 50, a first back reflector layer 60, and asecond back reflector layer 70. A first material junction is locatedbetween the p-doped first semiconductor material layer 30 and theintrinsic second semiconductor material layer 40′. A second materialjunction is located between the intrinsic second semiconductor materiallayer 40′ and the n-doped first semiconductor layer 50.

The substrate 10 is a structure that provides mechanical support to thephotovoltaic device structure, i.e., fifth hypothetical semiconductorstructure. The substrate 10 may be transparent in the range ofelectromagnetic radiation at which photogeneration of electrons andholes occur within the photovoltaic device structure. If thephotovoltaic device structure is a solar cell structure, the substrate10 can be optically transparent. The substrate 10 can be a glasssubstrate. The thickness of the substrate 10 can be from 50 microns to 3mm, although lesser and greater thicknesses can also be employed.

The contact material layer 20 includes a conductive material thatprovides electrical contact to the p-doped first semiconductor materiallayer 30. The contact material layer 20 can be a transparent conductivematerial layer that includes a material that is transparent in the rangeof electromagnetic radiation at which photogeneration of electrons andholes occur within the photovoltaic device structure. If thephotovoltaic device structure is employed as a solar cell, the contactmaterial layer 20 can be optically transparent. For example, the contactmaterial layer 20 can include a transparent conductive oxide such as afluorine-doped tin oxide (SnO₂:F), an aluminum-doped zinc oxide(ZnO:Al), or indium tin oxide. The thickness of the contact materiallayer 20 can be from 300 nm to 3 microns, although lesser and greaterthicknesses can also be employed. The contact material layer 20 iselectrically connected to the p-doped first semiconductor material layer30.

The p-doped first semiconductor material layer 30 includes an amorphous,microcrystalline, or epitaxial p-doped first semiconductor-containingmaterial. In some cases, the p-doped first semiconductor material layer30 can include a hydrogenated amorphous or microcrystalline p-dopedfirst semiconductor-containing material. The presence of hydrogen in thep-doped first semiconductor material layer 30 can increase the atomicconcentration of free charge carriers, i.e., holes, by delocalizing theelectrical charges that are pinned to defect sites. The firstsemiconductor material is selected in conjunction with a secondsemiconductor material such that the band gap of the first semiconductormaterial is greater than the band gap of the second semiconductormaterial. The dopant concentration of the p-type dopants can be constantthroughout the p-doped first semiconductor material layer 30.

The intrinsic second semiconductor material layer 40′ includes thesecond semiconductor material and does not include dopants. In oneembodiment, the intrinsic second semiconductor material layer 40′ canconsist of the second semiconductor material. Thus, the valence band andthe conduction band of the intrinsic second semiconductor material layer40′ are flat, and the Fermi level is located in the middle of thevalence band and the conduction band in the intrinsic secondsemiconductor material layer 40′.

The n-doped first semiconductor material layer 50 includes an amorphous,microcrystalline, or epitaxial n-doped first semiconductor-containingmaterial. In some cases, the n-doped first semiconductor material layer50 can include a hydrogenated amorphous or microcrystalline n-dopedfirst semiconductor-containing material. The presence of hydrogen in then-doped first semiconductor material layer 50 can increase the atomicconcentration of free charge carriers, i.e., holes, by delocalizing theelectrical charges that are pinned to defect sites. The dopantconcentration of the n-type dopants can be constant throughout then-doped first semiconductor material layer 50.

Referring to FIG. 8, a band diagram of the fifth hypotheticalsemiconductor structure is shown. The open circuit voltage of the fifthhypothetical semiconductor structure is affected by the external voltageto flatten either of the valence band or the conduction band. However,the bumps in the valence band or the conduction band reduce the opencircuit voltage of the fifth hypothetical semiconductor structure. Thus,the open circuit voltage of the fifth hypothetical semiconductorstructure does not significantly exceed the open circuit voltage of thesecond semiconductor material. In other words, the improvement in theopen circuit voltage of the fifth hypothetical semiconductor structurerelative a photovoltaic device employing a p-i-n junction in the secondsemiconductor material is limited due to the abrupt shifts in the energylevel in the valence band or in the conduction band.

Referring to FIG. 9, an exemplary semiconductor structure according tothe present disclosure further improves on the fifth hypotheticalsemiconductor structure by incorporating the feature of acompositionally-graded semiconductor material composition near a p-iinterface and an i-n interface. The exemplary semiconductor structure isa photovoltaic device structure that may be employed as a solar cellstructure. The intrinsic second semiconductor material layer 40′ in thefifth hypothetical semiconductor structure is replaced with an intrinsicsemiconductor material layer 40 in the exemplary semiconductorstructure. The intrinsic semiconductor material layer 40 includes thesecond semiconductor material and does not include dopants. In theexemplary semiconductor structure according to the present disclosure, afirst intrinsic compositionally-graded semiconductor layer 35, whichincludes a first compositionally-graded alloy of the first semiconductormaterial and the second semiconductor material, is present between ap-doped first semiconductor material layer 30 and an intrinsic secondsemiconductor material layer 40. Likewise, a second intrinsiccompositionally-graded semiconductor layer 45, which includes a secondcompositionally-graded alloy of the first semiconductor material and thesecond semiconductor material, is present between an n-doped firstsemiconductor material layer 50 and the intrinsic second semiconductormaterial layer 40.

In one embodiment, the intrinsic semiconductor material layer 40consists of the second semiconductor material. In this embodiment, theatomic percentage of the first semiconductor material in the firstintrinsic compositionally-graded semiconductor layer 35 typicallybecomes zero at the interface with the intrinsic material layer 40, andthe atomic percentage of the first semiconductor material in the secondintrinsic compositionally-graded semiconductor layer 45 typicallybecomes zero at the interface with the intrinsic material layer 40.

In another embodiment, the intrinsic semiconductor material layer 40includes a homogenous alloy of the first semiconductor material and thesecond semiconductor material. In this embodiment, the atomic percentageof the first semiconductor material in the first intrinsiccompositionally-graded semiconductor layer 35 typically becomes the sameas the atomic concentration of the first semiconductor material in theintrinsic semiconductor material layer 40 at the interface with theintrinsic semiconductor material layer 40. Likewise, the atomicpercentage of the first semiconductor material in the second intrinsiccompositionally-graded semiconductor layer 45 typically becomes the sameas the atomic concentration of the first semiconductor material in theintrinsic semiconductor material layer 40 at the interface with theintrinsic material layer 40. The atomic concentration of the firstsemiconductor material in the intrinsic semiconductor material layer 40can be from 0% to 99%, and typically from 0% to 50%, and preferably from0% to 20%. The valence band and the conduction band of the intrinsicsemiconductor material layer 40 are flat, and the Fermi level is locatedin the middle of the valence band and the conduction band in theintrinsic semiconductor material layer 40.

The material junctions between the first semiconductor material and thesecond semiconductor material in the fifth hypothetical semiconductorstructure are removed in the exemplary semiconductor structure accordingto the present disclosure, and are replaced with acompositionally-graded composition layers in which the atomicconcentration of the first semiconductor material and the secondsemiconductor material monotonically change complimentarily. In otherwords, the atomic composition of the first semiconductor materialmonotonically increases or decreases as the atomic composition of thesecond semiconductor material correspondingly monotonically decreases orincreases. Preferably, the atomic concentration of the firstsemiconductor material and the second semiconductor material strictlychange complimentarily. A p-i junction coincides with the materialjunction at the interface between the p-doped first semiconductormaterial layer 30 and the first intrinsic compositionally-gradedsemiconductor layer 35. An i-n junction coincides with the materialjunction at the interface between the second intrinsiccompositionally-graded semiconductor layer 45 and the n-doped firstsemiconductor material layer 50.

The first intrinsic compositionally-graded semiconductor layer 35includes an alloy of the first semiconductor material and the secondsemiconductor material having complimentarily compositionally-gradedatomic concentrations of the first and second semiconductor materials.Within the first intrinsic compositionally-graded semiconductor layer35, the atomic concentration of the second semiconductor materialmonotonically decreases with distance from the intrinsic second materiallayer 40. Preferably, the atomic concentration of the secondsemiconductor material strictly decreases with distance from theintrinsic second material layer 40 within the first intrinsiccompositionally-graded semiconductor layer 35.

The second intrinsic compositionally-graded semiconductor layer 45includes an alloy of the first semiconductor material and the secondsemiconductor material having complimentarily compositionally-gradedatomic concentrations of the first and second semiconductor materials.Within the second intrinsic compositionally-graded semiconductor layer45, the atomic concentration of the second semiconductor materialmonotonically decreases with distance from the intrinsic second materiallayer 40. Preferably, the atomic concentration of the secondsemiconductor material strictly decreases with distance from theintrinsic second material layer 40 within the second intrinsiccompositionally-graded semiconductor layer 45.

Thus, the exemplary semiconductor structure is a photovoltaic devicethat includes a stack of the first intrinsic compositionally-gradedsemiconductor layer 35 including the first compositionally-graded alloyof the first semiconductor material and the second semiconductormaterial, the intrinsic semiconductor material layer 40 including atleast the second semiconductor material, and the second intrinsiccompositionally-graded semiconductor layer 45 including the secondcompositionally-graded alloy of the first semiconductor material and thesecond semiconductor material. The atomic concentration of the secondsemiconductor material monotonically decreases with distance from aninterface with the intrinsic semiconductor layer 40 in the firstcompositionally-graded alloy and the second compositionally-gradedalloy.

The first semiconductor material and the second semiconductor materialare selected so that the first semiconductor material has a greater bandgap width than the second semiconductor material. The firstsemiconductor material and the second semiconductor material can beselected from, but is not limited to, silicon, germanium, III-V compoundsemiconductor materials, II-VI compound semiconductor materials, or anyother compound semiconductors known in the art. Each of the firstsemiconductor material and the second semiconductor material can be asingle elemental semiconductor or a single compound semiconductor, analloy of a plurality of elemental semiconductors, an alloy of aplurality of compound semiconductors, or an alloy of at least oneelemental semiconductor and at least one compound semiconductor. Forexample, the first semiconductor material can be silicon and the secondsemiconductor material can be germanium.

The first compositionally-graded alloy in the first intrinsiccompositionally-graded semiconductor layer 35, the second semiconductormaterial and/or the first semiconductor material in the intrinsicsemiconductor material layer 40, and the second compositionally-gradedalloy in the second intrinsic compositionally-graded semiconductor layer45 can be amorphous, polycrystalline, or single crystalline, i.e.,epitaxially aligned to one another. In one embodiment, the firstcompositionally-graded alloy, the semiconductor material(s) in theintrinsic semiconductor material layer 40, and the secondcompositionally-graded alloy may include hydrogenated amorphoussemiconductor materials.

In one embodiment, the atomic concentration of the second semiconductormaterial strictly decreases with distance from the interface with theintrinsic semiconductor layer 40 in the first compositionally-gradedalloy within the first intrinsic compositionally-graded semiconductorlayer 35 and the second compositionally-graded alloy within the secondintrinsic compositionally-graded semiconductor layer 45.

In one embodiment, the atomic concentration of the second semiconductormaterial can be 100% at a first interface between the first intrinsiccompositionally-graded semiconductor layer 35 and the intrinsicsemiconductor layer 40, and the atomic concentration of the secondsemiconductor material can be 100% at a second interface between thesecond intrinsic compositionally-graded semiconductor layer 45 and theintrinsic semiconductor layer 40. In this embodiment, the intrinsicsemiconductor material layer 40 typically consists of the secondsemiconductor material.

In one embodiment, the atomic concentration of the second semiconductormaterial can be 0% at a first end surface of the first intrinsiccompositionally-graded semiconductor layer 35, i.e., at an interfacewith the p-doped first semiconductor material layer 30. Likewise, theatomic concentration of the second semiconductor material can be 0% at asecond end surface of the second intrinsic compositionally-gradedsemiconductor layer 45, i.e., at an interface with the n-doped firstsemiconductor material layer 50.

The p-doped first semiconductor material layer 30 typically consists ofa p-doped first semiconductor material, i.e., the first semiconductormaterial and p-type dopants. The n-doped first semiconductor materiallayer 50 typically consists of an n-doped first semiconductor material,i.e., the first semiconductor material and n-type dopants.

The thickness of each layer in the exemplary semiconductor structure canbe optimized for performance. For example, the thickness of the p-dopedfirst semiconductor material layer 30 can be from 50 nm to 500 nm, thethickness of the first intrinsic compositionally-graded semiconductorlayer 35 can be from 10 nm to 200 nm, the thickness of the intrinsicsemiconductor material layer 40 can be from 10 nm to 400 nm, thethickness of the second intrinsic compositionally-graded semiconductorlayer 45 can be from 10 nm to 200 nm, and the thickness of the n-dopedfirst semiconductor material layer 50 can be from 50 nm to 500 nm,although lesser and greater thicknesses can also be employed.

The first back reflector layer 60 includes a transparent conductivematerial that is transparent in the range of electromagnetic radiationat which photogeneration of electrons and holes occur within thephotovoltaic device structure. If the photovoltaic device structure isemployed as a solar cell, the first back reflector layer 60 can beoptically transparent. For example, the first back reflector layer 60can include a transparent conductive oxide such as a fluorine-doped tinoxide (SnO₂:F), an aluminum-doped zinc oxide (ZnO:Al), or indium tinoxide. Since such transparent conductive oxide materials are n-typematerials, the contact between the first back reflector layer 60 and then-doped semiconductor layer 50 is Ohmic, and as such, the contactresistance between the first back reflector layer 60 and the n-dopedsemiconductor layer 50 is negligible. The thickness of the backreflector layer 60 can be from 25 nm to 250 nm, although lesser andgreater thicknesses can also be employed.

The second back reflector layer 70 includes a metallic material.Preferably, the metallic material has a high reflectivity in the rangeof electromagnetic radiation at which photogeneration of electrons andholes occur within the photovoltaic device structure. The metallicmaterial can include silver, aluminum, or an alloy thereof. Thethickness of the second back reflector layer 70 can be from 100 nm to 1micron, although lesser and greater thicknesses can also be employed.

The p-doped first semiconductor material layer 30 and the contactmaterial layer 20 are electrically connected to the first intrinsiccompositionally-graded semiconductor layer 35. The first back reflectorlayer 60 and the second back reflector layer 70 are electricallyconnected to the second intrinsic compositionally-graded semiconductorlayer 45 and the n-doped first semiconductor material layer 50. Thefirst back reflector layer 60 can be optically transparent, and thecombination of the first and second back reflector layers (60, 70)reflect any photons that pass through the stack of the p-doped firstsemiconductor material layer 30, the intrinsic semiconductor layer 140,and the n-doped first semiconductor material layer 50 to enhance theefficiency of the prior art photovoltaic device.

Referring to FIG. 10, a band diagram of the exemplary semiconductorstructure is shown. The open circuit voltage of the exemplarysemiconductor structure is determined by the external voltage to flatteneither of the valence band or the conduction band. Because bumps in thevalence band or the conduction band are eliminated in the exemplarysemiconductor structure relative to the fifth hypothetical semiconductorstructure, there is little loss in the open circuit voltage compared tothe open circuit voltage of a homogeneous photovoltaic device employingthe first semiconductor material and having a p-i-n junction or a p-njunction. Thus, the open circuit voltage of the exemplary semiconductorstructure is essentially the same as the open circuit voltage of such ahomogeneous photovoltaic device employing the first semiconductormaterial. At the same time, the short circuit current density isdetermined by the property of the material in the intrinsicsemiconductor material layer 40, i.e., the property of the secondsemiconductor material if the intrinsic semiconductor material layer 40consists of the second semiconductor material, or by the property of thealloy of the first and second semiconductor materials if the intrinsicsemiconductor material layer 40 includes an alloy of the first andsecond semiconductor materials. Thus, the exemplary semiconductorstructure can simultaneously provide the open circuit voltagesobtainable from the first material and the short circuit currentobtainable from the second material without suffering adverse effects.

FIG. 11A-11C are sequential vertical cross-sectional views thatillustrate a manufacturing process for forming the exemplarysemiconductor structure of FIG. 9. Referring to FIG. 11A, the substrate10 is provided, on which the contact material layer 20 is formed, forexample, by deposition. In one embodiment, the substrate 20 includes amaterial that is transparent in the range of electromagnetic radiationat which photogeneration of electrons and holes occur within thephotovoltaic structure as describe above. The contact material layer 20can include, for example, a transparent conductive material layer.

The p-doped first semiconductor material layer 30 can be deposited, forexample, by chemical vapor deposition (CVD) employing a first reactantincluding the first semiconductor material. P-type dopants can beintroduced into the p-doped first semiconductor material layer 30, forexample, by in-situ doping or by ex-situ doping such as ion implantationor plasma doping.

Referring to FIG. 11B, the first intrinsic compositionally-gradedsemiconductor layer 35 is deposited by introducing a second reactantincluding the second semiconductor material. The ratio of the flow rateof the second reactant to the flow rate of the first reactantmonotonically increases, and preferably strictly increases, during thedeposition of the first intrinsic compositionally-graded semiconductorlayer 35. In case the intrinsic semiconductor material layer 40 consistsof the second semiconductor material, the ratio can increase from zeroto infinity, i.e., the composition of the reactant can gradually changefrom the first reactant to the second reactant. In case the intrinsicsemiconductor material layer 40 includes an alloy of the first andsecond semiconductor materials, the ratio can increase from zero to afinite number during the deposition step.

The intrinsic semiconductor material layer 40 is deposited bymaintaining the same gas flow as at the end of the deposition step forthe first intrinsic compositionally-graded semiconductor layer 35. Nodopant gas is flowed into the process chamber during deposition of theintrinsic semiconductor material layer 40. In case the intrinsicsemiconductor material layer 40 consists of the second semiconductormaterial, only the second reactant is flowed into the process chamberduring the deposition step for the intrinsic semiconductor materiallayer 40. In case the intrinsic semiconductor material layer 40 includesan alloy of the first and second semiconductor materials, a mixture ofthe first and second reactant gases are flowed into the process chamber.

The second intrinsic compositionally-graded semiconductor layer 45 isdeposited by flowing a mixture of the first and second reactants suchthat the flow rate of the first reactant gradually increases during thedeposition of the second intrinsic compositionally-graded semiconductorlayer 45. The ratio of the flow rate of the second reactant to the flowrate of the first reactant monotonically decreases, and preferablystrictly decreases, during the deposition of the second intrinsiccompositionally-graded semiconductor layer 45. In one embodiment, onlythe first reactant flows at the end of this deposition step.

The n-doped first semiconductor material layer 50 can be deposited, forexample, by chemical vapor deposition (CVD) employing a first reactantincluding the first semiconductor material. N-type dopants can beintroduced into the n-doped first semiconductor material layer 50, forexample, by in-situ doping or by ex-situ doping such as ion implantationor plasma doping.

In general, the deposition of the p-doped first semiconductor materiallayer 30, the first intrinsic compositionally-graded semiconductor layer35, the intrinsic semiconductor material layer 40, the second intrinsiccompositionally-graded semiconductor layer 45, and the n-doped firstsemiconductor material layer 50 can be effected in a process chamberconfigured to flow the first and second reactants and a carrier gas. Thechemical vapor deposition process can be plasma enhanced chemical vaporprocess (PECVD) performed at a deposition temperature from 50° C. to400° C., and preferably from 100° C. to 350° C., and at a pressure from0.1 Torr to 10 Torr, and preferably from 0.2 Ton to 5 Ton.

Referring to FIG. 11C, the first back reflector layer 60 is deposited onthe n-doped semiconductor layer 50 employing methods known in the art.The first back reflector layer 60 includes a transparent conductivematerial. The second back reflector layer 70 is subsequently depositedon the first back reflector layer 70, for example, by electroplating,electroless plating, physical vapor deposition, chemical vapordeposition, vacuum evaporation, or a combination thereof. The secondback reflector layer 70 can be a metallic layer.

In a variation of the exemplary semiconductor structure, the dopingtypes can be reversed, i.e., the p-type doped structures in theexemplary semiconductor structure can be substituted with n-type dopedstructures and the n-type doped structures in the exemplarysemiconductor structure can be substituted with p-type doped structuresin this variation. In general, therefore, a doped first semiconductormaterial layer having a doping of a first conductivity type is formed onthe substrate 10. A first intrinsic compositionally-graded semiconductorlayer is formed on the doped first semiconductor material layer. Thefirst intrinsic compositionally-graded semiconductor layer has a dopingof the first conductivity type. If the first conductivity type isp-type, the first intrinsic compositionally-graded semiconductor layeris the first intrinsic compositionally-graded semiconductor layer 35. Ifthe first conductivity type is n-type, the first intrinsiccompositionally-graded semiconductor layer is a second intrinsiccompositionally-graded semiconductor layer, which is the same as thefirst intrinsic compositionally-graded semiconductor layer 35 exceptthat all p-type dopants are replaced with n-type dopants.

An intrinsic semiconductor material layer 40 is formed on the firstintrinsic compositionally-graded semiconductor layer. The intrinsicsemiconductor material layer 40 including at least the secondsemiconductor material as discussed above. A second intrinsiccompositionally-graded semiconductor layer is formed on the intrinsicsemiconductor material layer 40. The second intrinsiccompositionally-graded semiconductor layer has a doping of a secondconductivity type, which is the opposite of the first conductivity type.For example, if the first conductivity type is p-type, the secondconductivity type is n-type, and vice versa. If the first conductivitytype is p-type, the second intrinsic compositionally-gradedsemiconductor layer is the second intrinsic compositionally-gradedsemiconductor layer 45. If the first conductivity type is n-type, thefirst intrinsic compositionally-graded semiconductor layer is a firstintrinsic compositionally-graded semiconductor layer, which is the sameas the second intrinsic compositionally-graded semiconductor layer 45except that all n-type dopants are replaced with p-type dopants. A dopedsecond semiconductor material layer having a doping of the secondconductivity type is formed on the second intrinsiccompositionally-graded semiconductor layer 45. The first back reflectorlayer 60 and the second back reflector layer 70 can be formedthereafter.

While the present disclosure has been particularly shown and describedwith respect to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formsand details can be made without departing from the spirit and scope ofthe present disclosure. It is therefore intended that the presentdisclosure not be limited to the exact forms and details described andillustrated, but fall within the scope of the appended claims.

What is claimed is:
 1. A photovoltaic device comprising a stack, fromtop to bottom, of a first intrinsic compositionally-graded semiconductorlayer including a first compositionally-graded alloy of a firstsemiconductor material and a second semiconductor material, an intrinsicsemiconductor material layer including at least said secondsemiconductor material, and a second intrinsic compositionally-gradedsemiconductor layer including a second compositionally-graded alloy ofsaid first semiconductor material and said second semiconductormaterial, wherein an atomic concentration of said second semiconductormaterial monotonically decreases with distance from an interface withsaid intrinsic semiconductor layer in said first compositionally-gradedalloy and said second compositionally-graded alloy.
 2. The photovoltaicdevice of claim 1, wherein said first semiconductor material has agreater band gap width than said second semiconductor material.
 3. Thephotovoltaic device of claim 2, wherein said first semiconductormaterial is silicon and said second semiconductor material is germanium.4. The photovoltaic device of claim 2, wherein said firstcompositionally-graded alloy, said intrinsic semiconductor materiallayer, and said second compositionally-graded alloy are amorphous. 5.The photovoltaic device of claim 4, wherein said firstcompositionally-graded alloy, said intrinsic semiconductor materiallayer, and said second compositionally-graded alloy include hydrogenatedamorphous semiconductor materials.
 6. The photovoltaic device of claim2, wherein said first compositionally-graded alloy, said intrinsicsemiconductor material layer, and said second compositionally-gradedalloy are polycrystalline.
 7. The photovoltaic device of claim 2,wherein said first compositionally-graded alloy, said intrinsicsemiconductor material layer, and said second compositionally-gradedalloy are single crystalline and epitaxially aligned to one another. 8.The photovoltaic device of claim 1, wherein said atomic concentration ofsaid second semiconductor material strictly decreases with distance froman interface with said intrinsic semiconductor layer in said firstcompositionally-graded alloy and said second compositionally-gradedalloy.
 9. The photovoltaic device of claim 1, wherein said atomicconcentration of said second semiconductor material is 100% at a firstinterface between said first intrinsic compositionally-gradedsemiconductor layer and said intrinsic semiconductor layer, and saidatomic concentration of said second semiconductor material is 100% at asecond interface between said second intrinsic compositionally-gradedsemiconductor layer and said intrinsic semiconductor layer.
 10. Thephotovoltaic device of claim 9, wherein said atomic concentration ofsaid second semiconductor material is 0% at a first end surface of saidfirst intrinsic compositionally-graded semiconductor layer, and saidatomic concentration of said second semiconductor material is 0% at asecond end surface of said second intrinsic compositionally-gradedsemiconductor layer.
 11. The photovoltaic device of claim 10, furthercomprising: a p-doped first semiconductor material layer consisting of ap-doped first semiconductor material and located on said first endsurface, and an n-doped first semiconductor material layer consisting ofan n-doped first semiconductor material and located on said first endsurface.
 12. The photovoltaic device of claim 11, wherein said firstsemiconductor material is silicon and said second semiconductor materialis germanium.
 13. The photovoltaic device of claim 12, wherein saidfirst semiconductor material is amorphous hydrogenated silicon and saidsecond semiconductor material is amorphous hydrogenated germanium. 14.The photovoltaic device of claim 11, wherein a p-i junction iscoincident with a material junction between said p-doped firstsemiconductor material layer and said first intrinsiccompositionally-graded semiconductor layer.
 15. The photovoltaic deviceof claim 14, wherein an i-n junction is coincident with a materialjunction between said n-doped first semiconductor material layer andsaid second intrinsic compositionally-graded semiconductor layer. 16.The photovoltaic device of claim 1, wherein said first intrinsiccompositionally-graded semiconductor layer has a thickness from 10 nm to200 nm, said an intrinsic semiconductor material layer has a thicknessfrom 10 nm to 400 nm, and said second intrinsic compositionally-gradedsemiconductor layer has a thickness from 10 nm to 200 nm.
 17. Thephotovoltaic device of claim 1, further comprising; a transparentconductive material layer electrically connected to said first intrinsiccompositionally-graded semiconductor layer; and at least one backreflector layer electrically connected to said second intrinsiccompositionally-graded semiconductor layer.
 18. The photovoltaic deviceof claim 1, wherein said intrinsic semiconductor material layer consistsof said second semiconductor material.
 19. A method of forming aphotovoltaic device comprising: forming a first intrinsiccompositionally-graded semiconductor layer on a substrate; forming anintrinsic semiconductor material layer on said first intrinsiccompositionally-graded semiconductor layer, said intrinsic semiconductormaterial layer including at least said second semiconductor material;and forming a second intrinsic compositionally-graded semiconductorlayer on said intrinsic semiconductor material layer, said secondintrinsic compositionally-graded semiconductor layer including a secondcompositionally-graded alloy of said first semiconductor material andsaid second semiconductor material, wherein an atomic concentration ofsaid second semiconductor material monotonically decreases with distancefrom an interface with said intrinsic semiconductor layer in said firstcompositionally-graded alloy and said second compositionally-gradedalloy, and said second conductivity type is the opposite of said firstconductivity type.
 20. The method of claim 19, wherein said firstsemiconductor material has a greater band gap width than said secondsemiconductor material.
 21. The method of claim 20, wherein said firstcompositionally-graded alloy, said intrinsic semiconductor materiallayer, and said second compositionally-graded alloy are amorphous. 22.The method of claim 19, wherein said atomic concentration of said secondsemiconductor material strictly decreases with distance from aninterface with said intrinsic semiconductor layer in said firstcompositionally-graded alloy and said second compositionally-gradedalloy.
 23. The method of claim 19, wherein said atomic concentration ofsaid second semiconductor material is 100% at a first interface betweensaid first intrinsic compositionally-graded semiconductor layer and saidintrinsic semiconductor layer, and said atomic concentration of saidsecond semiconductor material is 100% at a second interface between saidsecond intrinsic compositionally-graded semiconductor layer and saidintrinsic semiconductor layer.
 24. The method of claim 19, furthercomprising: forming a doped first semiconductor material layerconsisting of said first semiconductor material and dopants of saidfirst conductivity type on said substrate, and forming another dopedfirst semiconductor material layer consisting of said firstsemiconductor material and dopants of said second conductivity type onsaid second intrinsic compositionally-graded semiconductor layer. 25.The method of claim 1, wherein said substrate is optically transparent,and said method further comprises; forming a transparent conductivematerial layer on said substrate; and forming at least one backreflector layer on said second intrinsic compositionally-gradedsemiconductor layer.